We have discovered potent natural secretory defenses in normal human milk (NHM) which are active in vitro against two important intestinal protozoan parasites: Giardia lamblia and Entamoeba histolytica. 1. NHM (from five donors) rapidly killed G. lamblia, but cow's milk and goat's mild did not. 2. The killing was not dependent upon secretory IgA. 3. The parasites were completely protected by bovine or human serum or serum albumin. 4. NHM has two Giardia-cidal activities, one thermolabile and one very heat stable. 5. Entamoeba histolytica and Trichomonas vaginalis were also killed by NHM. 6. It is likely that an unusual lipase is the thermolabile anti-parasitic factor in NHM. 7. The molecular target and mlde of Giardia-killing by lipases were explored.